效果图：

<el-table ref="courseList" :data="courseList" row-key="id" @selection-change="handleCourseSelection">
//...
<el-table>



//方法
handleCourseSelection(selection) {
            // console.log(selection);
        if(selection.length > 2) {
          this.$message.warning(`最多只能选择2个课程`);
          let del_row = selection.pop()
          this.$refs.courseList.toggleRowSelection(del_row, false)
        } else {
          this.selsectionCourse = selection
        }
      },


//在样式里面增加这个可以隐藏全选框
::v-deep .el-table__header-wrapper .el-checkbox {
  // display: none;//设置不成功，页面卡顿
  visibility: hidden;
}